1. Field of the Invention
The present invention relates to a heterojunction field effect transistor having an InP substrate.
2. Description of the Prior Art
Conventionally, in a heterojunction field effect transistor having an InP substrate, a structure is disclosed in, e.g., Japanese Unexamined Patent Publication No. 63-278277 issued on Nov. 15, 1988, which uses an InAlAs/InGaAs superlattice as the buffer layer, in order to prevent two-dimensional electron gas generation at an interface between a buffer layer and an active layer which occurs when only an InAlAs layer lattice-matched with an InP substrate is used as the buffer layer, and to prevent two-dimensional electron gas generation in an InGaAs layer in a superlattice which occurs when an InAlAs/InAlGaAs superlattice lattice-matched with an InP substrate is used as the buffer layer.
FIG. 1A shows the sectional structure of a field effect transistor disclosed in this official gazette, and FIG. 1B shows an energy band diagram indicating a change in conduction band of the field effect transistor.
Referring to FIG. 1A, reference numeral 611 denotes an InP substrate; 612, an InAlAs/InAlGaAs superlattice; 613, an InGaAs active layer; 614, an n-InAlAs electron supply layer; 615, an n-InGaAs contact layer; 616, a source electrode; 617, a gate electrode; and 618, a drain electrode.
In the structure shown in FIG. 1A, when compared to a structure using only an InAlAs layer which is lattice-matched with an InP substrate as the buffer layer, an impurity serving as a heavy donor is trapped in the heterointerface of the InAlAs/InAlGaAs superlattice 612 lattice-matched with the InP substrate 611. Thus, two-dimensional electron gas generation at the interface between the InGaAs active layer 613 and the InAlAs/InAlGaAs superlattice 612 serving as the buffer layer is suppressed.
When compared to a structure using an InAlAs/InGaAs superlattice lattice-matched with an InP substrate as the buffer layer, the InAlGaAs layer has a larger band gap than the InGaAs layer. Thus, two-dimensional electron gas generation in the InAlGaAs layer in the superlattice is decreased.
In the above conventional structure, however, when the GaAs composition ratio of the InAlGaAs layer is less than 0.87, as a difference in composition is small between the AlInAs layer and the InAlGaAs layer, the effect of suppressing propagation of dislocations present in the InP substrate to the active layer, diffusion of an impurity, e.g., Fe, doped in increase the resistance of the InP substrate, and the like is small. Thus, the crystallinity of the active layer is degraded, leading to a low transconductance of the transistor.
The AlInAs layer is of an n type of about 1.times.10.sup.15 /cm.sup.3. In the conventional structure, when the GaAs composition ratio of the InAlGaAs layer is higher than 0.87 molar fraction, the band gap of the InAlGaAs layer becomes smaller than that of the InP substrate. If the film thickness of the AlInAs/InAlGaAs superlattice buffer layer is set to a value sufficient for eliminating propagation of the dislocations present in the InP substrate to the active layer, diffusion of the impurity, e.g., Fe, doped in increase the resistance of the InP substrate, and the like, electrons are generated in the AlInAs/InAlGaAs superlattice buffer layer. Then, an electron conduction path is formed in addition to the active layer, leading to a degradation in the pinch-off characteristics of the transistor.